Traditional histology methods for preserving tissues for microscopy or other forms of examination generally require days to complete. Tissue processing typically involves at least the following steps: (i) fixation, (ii) dehydration, (iii) clearing, and (iv) infiltration with embedding medium. Rapid tissue processing methods utilizing microwave energy were introduced in the late 1980's, for example, as taught in U.S. Pat. No. 4,656,047 to Kok et al., U.S. Pat. No. 4,839,194 to Malluche et al. and U.S. Pat. No. 4,891,239 to Dudley et al. Instruments for performing automated microwave processing are taught in U.S. Pat. No. 4,891,239 and U.S. Pat. No. 6,207,408, among others. Such instruments have been made commercially available and the practice of rapid tissue processing methods is increasing.
Some of the procedures for rapid processing combine steps, for example, simultaneous fixation and dehydration, as taught in U.S. Pat. No. 6,207,408 or simultaneous dehydration and clearing, as described in U.S. Pat. No. 6,042,874. Rapid processing methods utilizing microwave radiation are considered to preclude the need for traditional formaldehyde-containing fixation solutions, also called additive fixation solutions. Instead, fixation in these methods uses fixation solutions containing alcohols, ketones, acids, etc., also called non-additive fixation solutions.
Microwave-assisted processing, however, can be associated with artifacts, particularly when combined with non-additive fixation solution. Some automated microwave-based tissue processors limit the maximum thickness of tissue specimens to about 2.5 mm. To date, instruments for performing automated microwave-assisted processing have been expensive. There is thus a need for an affordable rapid tissue processing system capable of preserving tissue specimens up to 5 mm thick and that more closely follows traditional formaldehyde-based (i.e., additive) methods.